Measuring servo reader span on a magnetic recording head

ABSTRACT

A computer-implemented method for measuring servo to servo reader spacing in a module, according to one embodiment, includes cause emission of a measurable electro-magnetic signal from two writer elements that are fixed at a same span as a nominal servo reader spacing. The writer elements are positioned in multiple relative positions along a transducer array of a module having servo readers in the transducer array. The strength of the electro-magnetic signal detected by each servo reader is measured at the multiple relative positions. A determination is made, from the measured strengths, whether the servo reader span matches the nominal servo reader spacing.

BACKGROUND

The present invention relates to data storage systems, and more particularly, this invention relates to measuring servo reader span on a magnetic recording head.

In magnetic storage systems, magnetic transducers read data from and write data onto magnetic recording media. Data is written on the magnetic recording media by moving a magnetic recording transducer to a position over the media where the data is to be stored. The magnetic recording transducer then generates a magnetic field, which encodes the data into the magnetic media. Data is read from the media by similarly positioning the magnetic read transducer and then sensing the magnetic field of the magnetic media. Read and write operations may be independently synchronized with the movement of the media to ensure that the data can be read from and written to the desired location on the media.

An important and continuing goal in the data storage industry is that of increasing the density of data stored on a medium. For tape storage systems, that goal has led to increasing the track and linear bit density on recording tape, and decreasing the thickness of the magnetic tape medium. However, the development of small footprint, higher performance tape drive systems has created various challenges ranging from the design of tape head assemblies for use in such systems to dealing with tape dimensional instability.

SUMMARY

A computer-implemented method for measuring servo to servo reader spacing in a module, according to one embodiment, includes cause emission of a measurable electro-magnetic signal from two writer elements that are fixed at a same span as a nominal servo reader spacing. The writer elements are positioned in multiple relative positions along a transducer array of a module having servo readers in the transducer array. The strength of the electro-magnetic signal detected by each servo reader is measured at the multiple relative positions. A determination is made, from the measured strengths, whether the servo reader span matches the nominal servo reader spacing.

An apparatus, according to one embodiment, includes a processor, and logic integrated with the processor, executable by the processor, or integrated with and executable by the processor. The logic is configured to: cause two writer elements that are fixed at a same span as a nominal servo reader spacing to emit a measurable electro-magnetic signal. The logic is also configured to position the writer elements in multiple relative positions along a transducer array of a module having servo readers in the transducer array. The logic is also configured to measure the strength of the electro-magnetic signal detected by each servo reader at the multiple relative positions. The logic is also configured to determine, from the measured strengths, whether the servo reader matches the nominal servo reader spacing.

A computer program product for measuring servo to servo reader spacing in a module includes a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a computer to cause the computer to perform the foregoing method.

Other aspects and embodiments of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a simplified tape drive system according to one embodiment.

FIG. 1B is a schematic diagram of a tape cartridge according to one embodiment.

FIG. 2A illustrates a side view of a flat-lapped, bi-directional, two-module magnetic tape head according to one embodiment.

FIG. 2B is a tape bearing surface view taken from Line 2B of FIG. 2A.

FIG. 2C is a detailed view taken from Circle 2C of FIG. 2B.

FIG. 2D is a detailed view of a partial tape bearing surface of a pair of modules.

FIG. 3 is a partial tape bearing surface view of a magnetic head having a write-read-write configuration.

FIG. 4 is a partial tape bearing surface view of a magnetic head having a read-write-read configuration.

FIG. 5 is a side view of a magnetic tape head with three modules according to one embodiment where the modules all generally lie along about parallel planes.

FIG. 6 is a side view of a magnetic tape head with three modules in a tangent (angled) configuration.

FIG. 7 is a side view of a magnetic tape head with three modules in an overwrap configuration.

FIGS. 8A-8C are schematics depicting the principles of tape tenting.

FIG. 9 is a representational diagram of files and indexes stored on a magnetic tape according to one embodiment.

FIG. 10 is a side view of a pair of modules, in accordance with one embodiment.

FIG. 11 is a side view of a pair of modules, in accordance with one embodiment.

FIGS. 12A-12C graphically depict a method for measuring servo to servo reader spacing in a module, in accordance with one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.

The following description discloses several preferred embodiments of systems for testing components of magnetic storage systems, as well as operation thereof.

In one general embodiment, a computer-implemented method for measuring servo to servo reader spacing in a module includes cause emission of a measurable electro-magnetic signal from two writer elements that are fixed at a same span as a nominal servo reader spacing. The writer elements are positioned in multiple relative positions along a transducer array of a module having servo readers in the transducer array. The strength of the electro-magnetic signal detected by each servo reader is measured at the multiple relative positions. A determination is made, from the measured strengths, whether the servo reader span matches the nominal servo reader spacing.

In another general embodiment, an apparatus includes a processor, and logic integrated with the processor, executable by the processor, or integrated with and executable by the processor. The logic is configured to: cause two writer elements that are fixed at a same span as a nominal servo reader spacing to emit a measurable electro-magnetic signal. The logic is also configured to position the writer elements in multiple relative positions along a transducer array of a module having servo readers in the transducer array. The logic is also configured to measure the strength of the electro-magnetic signal detected by each servo reader at the multiple relative positions. The logic is also configured to determine, from the measured strengths, whether the servo reader matches the nominal servo reader spacing.

In yet another general embodiment, a computer program product for measuring servo to servo reader spacing in a module includes a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a computer to cause the computer to perform the foregoing method.

FIG. 1A illustrates a simplified tape drive 100 of a tape-based data storage system, which may be employed in the context of the present invention. While one specific implementation of a tape drive is shown in FIG. 1A, it should be noted that the embodiments described herein may be implemented in the context of any type of tape drive system.

As shown, a tape supply cartridge 120 and a take-up reel 121 are provided to support a tape 122. One or more of the reels may form part of a removable cartridge and are not necessarily part of the tape drive 100. The tape drive, such as that illustrated in FIG. 1A, may further include drive motor(s) to drive the tape supply cartridge 120 and the take-up reel 121 to move the tape 122 over a tape head 126 of any type. Such head may include an array of readers, writers, or both.

Guides 125 guide the tape 122 across the tape head 126. Such tape head 126 is in turn coupled to a controller 128 via a cable 130. The controller 128, may be or include a processor and/or any logic for controlling any subsystem of the drive 100. For example, the controller 128 typically controls head functions such as servo following, data writing, data reading, etc. The controller 128 may include at least one servo channel and at least one data channel, each of which include data flow processing logic configured to process and/or store information to be written to and/or read from the tape 122. The controller 128 may operate under logic known in the art, as well as any logic disclosed herein, and thus may be considered as a processor for any of the descriptions of tape drives included herein, in various embodiments. The controller 128 may be coupled to a memory 136 of any known type, which may store instructions executable by the controller 128. Moreover, the controller 128 may be configured and/or programmable to perform or control some or all of the methodology presented herein. Thus, the controller 128 may be considered to be configured to perform various operations by way of logic programmed into one or more chips, modules, and/or blocks; software, firmware, and/or other instructions being available to one or more processors; etc., and combinations thereof.

The cable 130 may include read/write circuits to transmit data to the tape head 126 to be recorded on the tape 122 and to receive data read by the tape head 126 from the tape 122. An actuator 132 controls position of the tape head 126 relative to the tape 122.

An interface 134 may also be provided for communication between the tape drive 100 and a host (internal or external) to send and receive the data and for controlling the operation of the tape drive 100 and communicating the status of the tape drive 100 to the host, all as will be understood by those of skill in the art.

FIG. 1B illustrates an exemplary tape cartridge 150 according to one embodiment. Such tape cartridge 150 may be used with a system such as that shown in FIG. 1A. As shown, the tape cartridge 150 includes a housing 152, a tape 122 in the housing 152, and a nonvolatile memory 156 coupled to the housing 152. In some approaches, the nonvolatile memory 156 may be embedded inside the housing 152, as shown in FIG. 1B. In more approaches, the nonvolatile memory 156 may be attached to the inside or outside of the housing 152 without modification of the housing 152. For example, the nonvolatile memory may be embedded in a self-adhesive label 154. In one preferred embodiment, the nonvolatile memory 156 may be a Flash memory device, read-only memory (ROM) device, etc., embedded into or coupled to the inside or outside of the tape cartridge 150. The nonvolatile memory is accessible by the tape drive and the tape operating software (the driver software), and/or another device.

By way of example, FIG. 2A illustrates a side view of a flat-lapped, bi-directional, two-module magnetic tape head 200 which may be implemented in the context of the present invention. As shown, the head includes a pair of bases 202, each equipped with a module 204, and fixed at a small angle α with respect to each other. The bases may be “U-beams” that are adhesively coupled together. Each module 204 includes a substrate 204A and a closure 204B with a thin film portion, commonly referred to as a “gap” in which the readers and/or writers 206 are formed. In use, a tape 208 is moved over the modules 204 along a media (tape) bearing surface 209 in the manner shown for reading and writing data on the tape 208 using the readers and writers. The wrap angle θ of the tape 208 at edges going onto and exiting the flat media support surfaces 209 are usually between about 0.1 degree and about 3 degrees.

The substrates 204A are typically constructed of a wear resistant material, such as a ceramic. The closures 204B may be made of the same or similar ceramic as the substrates 204A.

The readers and writers may be arranged in a piggyback or merged configuration. An illustrative piggybacked configuration comprises a (magnetically inductive) writer transducer on top of (or below) a (magnetically shielded) reader transducer (e.g., a magnetoresistive reader, etc.), wherein the poles of the writer and the shields of the reader are generally separated. An illustrative merged configuration comprises one reader shield in the same physical layer as one writer pole (hence, “merged”). The readers and writers may also be arranged in an interleaved configuration. Alternatively, each array of channels may be readers or writers only. Any of these arrays may contain one or more servo track readers for reading servo data on the medium.

FIG. 2B illustrates the tape bearing surface 209 of one of the modules 204 taken from Line 2B of FIG. 2A. A representative tape 208 is shown in dashed lines. The module 204 is preferably long enough to be able to support the tape as the head steps between data bands.

In this example, the tape 208 includes 4 to 32 data bands, e.g., with 16 data bands and 17 servo tracks 210, as shown in FIG. 2B on a one-half inch wide tape 208. The data bands are defined between servo tracks 210. Each data band may include a number of data tracks, for example 1024 data tracks (not shown). During read/write operations, the readers and/or writers 206 are positioned to specific track positions within one of the data bands. Outer readers, sometimes called servo readers, read the servo tracks 210. The servo signals are in turn used to keep the readers and/or writers 206 aligned with a particular set of tracks during the read/write operations.

FIG. 2C depicts a plurality of readers and/or writers 206 formed in a gap 218 on the module 204 in Circle 2C of FIG. 2B. As shown, the array of readers and writers 206 includes, for example, 16 writers 214, 16 readers 216 and two servo readers 212, though the number of elements may vary. Illustrative embodiments include 8, 16, 32, 40, and 64 active readers and/or writers 206 per array, and alternatively interleaved designs having odd numbers of reader or writers such as 17, 25, 33, etc. An illustrative embodiment includes 32 readers per array and/or 32 writers per array, where the actual number of transducer elements could be greater, e.g., 33, 34, etc. This allows the tape to travel more slowly, thereby reducing speed-induced tracking and mechanical difficulties and/or execute fewer “wraps” to fill or read the tape. While the readers and writers may be arranged in a piggyback configuration as shown in FIG. 2C, the readers 216 and writers 214 may also be arranged in an interleaved configuration. Alternatively, each array of readers and/or writers 206 may be readers or writers only, and the arrays may contain one or more servo readers 212. As noted by considering FIGS. 2A and 2B-2C together, each module 204 may include a complementary set of readers and/or writers 206 for such things as bi-directional reading and writing, read-while-write capability, backward compatibility, etc.

FIG. 2D shows a partial tape bearing surface view of complementary modules of a magnetic tape head 200 according to one embodiment. In this embodiment, each module has a plurality of read/write (R/W) pairs in a piggyback configuration formed on a common substrate 204A and an optional electrically insulative insulating layer 236. The writers 214 and the readers 216 are aligned parallel to an intended direction of travel of a tape medium thereacross to form an R/W pair, exemplified by R/W pairs 222. Note that the intended direction of tape travel is sometimes referred to herein as the direction of tape travel, and such terms may be used interchangeably. Such direction of tape travel may be inferred from the design of the system, e.g., by examining the guides; observing the actual direction of tape travel relative to the reference point; etc. Moreover, in a system operable for bi-direction reading and/or writing, the direction of tape travel in both directions is typically parallel and thus both directions may be considered equivalent to each other.

Several R/W pairs 222 may be present, such as 8, 16, 32 pairs, etc. The R/W pairs 222 as shown are linearly aligned in a direction generally perpendicular to a direction of tape travel thereacross. However, the pairs may also be aligned diagonally, etc. Servo readers 212 are positioned on the outside of the array of R/W pairs, the function of which is well known.

Generally, the magnetic tape medium moves in either a forward or reverse direction as indicated by arrow 220. The magnetic tape medium and head assembly 200 operate in a transducing relationship in the manner well-known in the art. The head assembly 200 includes two thin-film modules 224 and 226 of generally identical construction.

Modules 224 and 226 are joined together with a space present between closures 204B thereof (partially shown) to form a single physical unit to provide read-while-write capability by activating the writer of the leading module and reader of the trailing module aligned with the writer of the leading module parallel to the direction of tape travel relative thereto. When a module 224, 226 of a magnetic tape head 200 is constructed, layers are formed in the gap 218 created above an electrically conductive substrate 204A (partially shown), e.g., of AlTiC, in generally the following order for the R/W pairs 222: an insulating layer 236, a first shield 232 typically of an iron alloy such as NiFe (—), cobalt zirconium tantalum (CZT) or Al—Fe—Si (Sendust), a sensor 234 for sensing a data track on a magnetic medium, a second shield 238 typically of a nickel-iron alloy (e.g., ˜80/20 at % NiFe, also known as permalloy), first and second writer poles 228, 230, and a coil (not shown). The sensor may be of any known type, including those based on magnetoresistive (MR), GMR, AMR, tunneling magnetoresistance (TMR), etc.

The first and second writer poles 228, 230 may be fabricated from high magnetic moment materials such as ˜45/55 NiFe. Note that these materials are provided by way of example only, and other materials may be used. Additional layers such as insulation between the shields and/or pole tips and an insulation layer surrounding the sensor may be present. Illustrative materials for the insulation include alumina and other oxides, insulative polymers, etc.

The configuration of the tape head 126 according to one embodiment includes multiple modules, preferably three or more. In a write-read-write (W-R-W) head, outer modules for writing flank one or more inner modules for reading. Referring to FIG. 3, depicting a W-R-W configuration, the outer modules 252, 256 each include one or more arrays of writers 260. The inner module 254 of FIG. 3 includes one or more arrays of readers 258 in a similar configuration. Variations of a multi-module head include a R-W-R head (FIG. 4), a R-R-W head, a W-W-R head, etc. In yet other variations, one or more of the modules may have read/write pairs of transducers. Moreover, more than three modules may be present. In further approaches, two outer modules may flank two or more inner modules, e.g., in a W-R-R-W, a R-W-W-R arrangement, etc. For simplicity, a W-R-W head is used primarily herein to exemplify embodiments of the present invention. One skilled in the art apprised with the teachings herein will appreciate how permutations of the present invention would apply to configurations other than a W-R-W configuration.

FIG. 5 illustrates a magnetic head 126 according to one embodiment of the present invention that includes first, second and third modules 302, 304, 306 each having a tape bearing surface 308, 310, 312 respectively, which may be flat, contoured, etc. Note that while the term “tape bearing surface” appears to imply that the surface facing the tape 315 is in physical contact with the tape bearing surface, this is not necessarily the case. Rather, only a portion of the tape may be in contact with the tape bearing surface, constantly or intermittently, with other portions of the tape riding (or “flying”) above the tape bearing surface on a layer of air, sometimes referred to as an “air bearing”. The first module 302 will be referred to as the “leading” module as it is the first module encountered by the tape in a three module design for tape moving in the indicated direction. The third module 306 will be referred to as the “trailing” module. The trailing module follows the middle module and is the last module seen by the tape in a three module design. The leading and trailing modules 302, 306 are referred to collectively as outer modules. Also note that the outer modules 302, 306 will alternate as leading modules, depending on the direction of travel of the tape 315.

In one embodiment, the tape bearing surfaces 308, 310, 312 of the first, second and third modules 302, 304, 306 lie on about parallel planes (which is meant to include parallel and nearly parallel planes, e.g., between parallel and tangential as in FIG. 6), and the tape bearing surface 310 of the second module 304 is above the tape bearing surfaces 308, 312 of the first and third modules 302, 306. As described below, this has the effect of creating the desired wrap angle α₂ of the tape relative to the tape bearing surface 310 of the second module 304.

Where the tape bearing surfaces 308, 310, 312 lie along parallel or nearly parallel yet offset planes, intuitively, the tape should peel off of the tape bearing surface 308 of the leading module 302. However, the vacuum created by a skiving edge 318 of the leading module 302 has been found by experimentation to be sufficient to keep the tape adhered to the tape bearing surface 308 of the leading module 302. A trailing edge 320 of the leading module 302 (the end from which the tape leaves the leading module 302) is the approximate reference point which defines the wrap angle α₂ over the tape bearing surface 310 of the second module 304. The tape stays in close proximity to the tape bearing surface until close to the trailing edge 320 of the leading module 302. Accordingly, transducers 322 may be located near the trailing edges of the outer modules 302, 306. These embodiments are particularly adapted for write-read-write applications.

A benefit of this and other embodiments described herein is that, because the outer modules 302, 306 are fixed at a determined offset from the second module 304, the inner wrap angle α₂ is fixed when the modules 302, 304, 306 are coupled together or are otherwise fixed into a head. The inner wrap angle α₂ is approximately tan⁻¹(δ/W) where δ is the height difference between the planes of the tape bearing surfaces 308, 310 and W is the width between the opposing ends of the tape bearing surfaces 308, 310. An illustrative inner wrap angle α₂ is in a range of about 0.3° to about 1.1°, though can be any angle required by the design.

Beneficially, the inner wrap angle α₂ on the side of the module 304 receiving the tape (leading edge) will be larger than the inner wrap angle α₃ on the trailing edge, as the tape 315 rides above the trailing module 306. This difference is generally beneficial as a smaller α₃ tends to oppose what has heretofore been a steeper exiting effective wrap angle.

Note that the tape bearing surfaces 308, 312 of the outer modules 302, 306 are positioned to achieve a negative wrap angle at the trailing edge 320 of the leading module 302. This is generally beneficial in helping to reduce friction due to contact with the trailing edge 320, provided that proper consideration is given to the location of the crowbar region that forms in the tape where it peels off the head. This negative wrap angle also reduces flutter and scrubbing damage to the elements on the leading module 302. Further, at the trailing module 306, the tape 315 flies over the tape bearing surface 312 so there is virtually no wear on the elements when tape is moving in this direction. Particularly, the tape 315 entrains air and so will not significantly ride on the tape bearing surface 312 of the third module 306 (some contact may occur). This is permissible, because the leading module 302 is writing while the trailing module 306 is idle.

Writing and reading functions are performed by different modules at any given time. In one embodiment, the second module 304 includes a plurality of data and optional servo readers 331 and no writers. The first and third modules 302, 306 include a plurality of writers 322 and no data readers, with the exception that the outer modules 302, 306 may include optional servo readers. The servo readers may be used to position the head during reading and/or writing operations. The servo reader(s) on each module are typically located towards the end of the array of readers or writers.

By having only readers or side by side writers and servo readers in the gap between the substrate and closure, the gap length can be substantially reduced. Typical heads have piggybacked readers and writers, where the writer is formed above each reader. A typical gap is 20-35 microns. However, irregularities on the tape may tend to droop into the gap and create gap erosion. Thus, the smaller the gap is the better. The smaller gap enabled herein exhibits fewer wear related problems.

In some embodiments, the second module 304 has a closure, while the first and third modules 302, 306 do not have a closure. Where there is no closure, preferably a hard coating is added to the module. One preferred coating is diamond-like carbon (DLC).

In the embodiment shown in FIG. 5, the first, second, and third modules 302, 304, 306 each have a closure 332, 334, 336, which extends the tape bearing surface of the associated module, thereby effectively positioning the read/write elements away from the edge of the tape bearing surface. The closure 332 on the second module 304 can be a ceramic closure of a type typically found on tape heads. The closures 334, 336 of the first and third modules 302, 306, however, may be shorter than the closure 332 of the second module 304 as measured parallel to a direction of tape travel over the respective module. This enables positioning the modules closer together. One way to produce shorter closures 334, 336 is to lap the standard ceramic closures of the second module 304 an additional amount. Another way is to plate or deposit thin film closures above the elements during thin film processing. For example, a thin film closure of a hard material such as Sendust or nickel-iron alloy (e.g., 45/55) can be formed on the module.

With reduced-thickness ceramic or thin film closures 334, 336 or no closures on the outer modules 302, 306, the write-to-read gap spacing can be reduced to less than about 1 mm, e.g., about 0.75 mm, or 50% less than commonly-used linear tape open (LTO) tape head spacing. The open space between the modules 302, 304, 306 can still be set to approximately 0.5 to 0.6 mm, which in some embodiments is ideal for stabilizing tape motion over the second module 304.

Depending on tape tension and stiffness, it may be desirable to angle the tape bearing surfaces of the outer modules relative to the tape bearing surface of the second module. FIG. 6 illustrates an embodiment where the modules 302, 304, 306 are in a tangent or nearly tangent (angled) configuration. Particularly, the tape bearing surfaces of the outer modules 302, 306 are about parallel to the tape at the desired wrap angle α₂ of the second module 304. In other words, the planes of the tape bearing surfaces 308, 312 of the outer modules 302, 306 are oriented at about the desired wrap angle α₂ of the tape 315 relative to the second module 304. The tape will also pop off of the trailing module 306 in this embodiment, thereby reducing wear on the elements in the trailing module 306. These embodiments are particularly useful for write-read-write applications. Additional aspects of these embodiments are similar to those given above.

Typically, the tape wrap angles may be set about midway between the embodiments shown in FIGS. 5 and 6.

FIG. 7 illustrates an embodiment where the modules 302, 304, 306 are in an overwrap configuration. Particularly, the tape bearing surfaces 308, 312 of the outer modules 302, 306 are angled slightly more than the tape 315 when set at the desired wrap angle α₂ relative to the second module 304. In this embodiment, the tape does not pop off of the trailing module, allowing it to be used for writing or reading. Accordingly, the leading and middle modules can both perform reading and/or writing functions while the trailing module can read any just-written data. Thus, these embodiments are preferred for write-read-write, read-write-read, and write-write-read applications. In the latter embodiments, closures should be wider than the tape canopies for ensuring read capability. The wider closures may require a wider gap-to-gap separation. Therefore, a preferred embodiment has a write-read-write configuration, which may use shortened closures that thus allow closer gap-to-gap separation.

Additional aspects of the embodiments shown in FIGS. 6 and 7 are similar to those given above.

A 32 channel version of a multi-module tape head 126 may use cables 350 having leads on the same or smaller pitch as current 16 channel piggyback LTO modules, or alternatively the connections on the module may be organ-keyboarded for a 50% reduction in cable span. Over-under, writing pair unshielded cables may be used for the writers, which may have integrated servo readers.

The outer wrap angles α₁ may be set in the drive, such as by guides of any type known in the art, such as adjustable rollers, slides, etc. or alternatively by outriggers, which are integral to the head. For example, rollers having an offset axis may be used to set the wrap angles. The offset axis creates an orbital arc of rotation, allowing precise alignment of the wrap angle α₁.

To assemble any of the embodiments described above, conventional u-beam assembly can be used. Accordingly, the mass of the resultant head may be maintained or even reduced relative to heads of previous generations. In other approaches, the modules may be constructed as a unitary body. Those skilled in the art, armed with the present teachings, will appreciate that other known methods of manufacturing such heads may be adapted for use in constructing such heads. Moreover, unless otherwise specified, processes and materials of types known in the art may be adapted for use in various embodiments in conformance with the teachings herein, as would become apparent to one skilled in the art upon reading the present disclosure.

As a tape is run over a module, it is preferred that the tape passes sufficiently close to magnetic transducers on the module such that reading and/or writing is efficiently performed, e.g., with a low error rate. According to some approaches, tape tenting may be used to ensure the tape passes sufficiently close to the portion of the module having the magnetic transducers. To better understand this process, FIGS. 8A-8C illustrate the principles of tape tenting. FIG. 8A shows a module 800 having an upper tape bearing surface 802 extending between opposite edges 804, 806. A stationary tape 808 is shown wrapping around the edges 804, 806. As shown, the bending stiffness of the tape 808 lifts the tape off of the tape bearing surface 802. Tape tension tends to flatten the tape profile, as shown in FIG. 8A. Where tape tension is minimal, the curvature of the tape is more parabolic than shown.

FIG. 8B depicts the tape 808 in motion. The leading edge, i.e., the first edge the tape encounters when moving, may serve to skive air from the tape, thereby creating a subambient air pressure between the tape 808 and the tape bearing surface 802. In FIG. 8B, the leading edge is the left edge and the right edge is the trailing edge when the tape is moving left to right. As a result, atmospheric pressure above the tape urges the tape toward the tape bearing surface 802, thereby creating tape tenting proximate each of the edges. The tape bending stiffness resists the effect of the atmospheric pressure, thereby causing the tape tenting proximate both the leading and trailing edges. Modeling predicts that the two tents are very similar in shape.

FIG. 8C depicts how the subambient pressure urges the tape 808 toward the tape bearing surface 802 even when a trailing guide 810 is positioned above the plane of the tape bearing surface.

It follows that tape tenting may be used to direct the path of a tape as it passes over a module. As previously mentioned, tape tenting may be used to ensure the tape passes sufficiently close to the portion of the module having the magnetic transducers, preferably such that reading and/or writing is efficiently performed, e.g., with a low error rate.

Magnetic tapes may be stored in tape cartridges that are, in turn, stored at storage slots or the like inside a data storage library. The tape cartridges may be stored in the library such that they are accessible for physical retrieval. In addition to magnetic tapes and tape cartridges, data storage libraries may include data storage drives that store data to, and/or retrieve data from, the magnetic tapes. Moreover, tape libraries and the components included therein may implement a file system which enables access to tape and data stored on the tape.

File systems may be used to control how data is stored in, and retrieved from, memory. Thus, a file system may include the processes and data structures that an operating system uses to keep track of files in memory, e.g., the way the files are organized in memory. Linear Tape File System (LTFS) is an exemplary format of a file system that may be implemented in a given library in order to enables access to compliant tapes. It should be appreciated that various embodiments herein can be implemented with a wide range of file system formats, including for example IBM Spectrum Archive Library Edition (LTFS LE). However, to provide a context, and solely to assist the reader, some of the embodiments below may be described with reference to LTFS which is a type of file system format. This has been done by way of example only, and should not be deemed limiting on the invention defined in the claims.

A tape cartridge may be “loaded” by inserting the cartridge into the tape drive, and the tape cartridge may be “unloaded” by removing the tape cartridge from the tape drive. Once loaded in a tape drive, the tape in the cartridge may be “threaded” through the drive by physically pulling the tape (the magnetic recording portion) from the tape cartridge, and passing it above a magnetic head of a tape drive. Furthermore, the tape may be attached on a take-up reel (e.g., see 121 of FIG. 1A above) to move the tape over the magnetic head.

Once threaded in the tape drive, the tape in the cartridge may be “mounted” by reading metadata on a tape and bringing the tape into a state where the LTFS is able to use the tape as a constituent component of a file system. Moreover, in order to “unmount” a tape, metadata is preferably first written on the tape (e.g., as an index), after which the tape may be removed from the state where the LTFS is allowed to use the tape as a constituent component of a file system. Finally, to “unthread” the tape, the tape is unattached from the take-up reel and is physically placed back into the inside of a tape cartridge again. The cartridge may remain loaded in the tape drive even after the tape has been unthreaded, e.g., waiting for another read and/or write request. However, in other instances, the tape cartridge may be unloaded from the tape drive upon the tape being unthreaded, e.g., as described above.

Magnetic tape is a sequential access medium. Thus, new data is written to the tape by appending the data at the end of previously written data. It follows that when data is recorded in a tape having only one partition, metadata (e.g., allocation information) is continuously appended to an end of the previously written data as it frequently updates and is accordingly rewritten to tape. As a result, the rearmost information is read when a tape is first mounted in order to access the most recent copy of the metadata corresponding to the tape. However, this introduces a considerable amount of delay in the process of mounting a given tape.

To overcome this delay caused by single partition tape mediums, the LTFS format includes a tape that is divided into two partitions, which include an index partition and a data partition. The index partition may be configured to record metadata (meta information), e.g., such as file allocation information (Index), while the data partition may be configured to record the body of the data, e.g., the data itself.

Looking to FIG. 9, a magnetic tape 900 having an index partition 902 and a data partition 904 is illustrated according to one embodiment. As shown, data files and indexes are stored on the tape. The LTFS format allows for index information to be recorded in the index partition 902 at the beginning of tape 906, as would be appreciated by one skilled in the art upon reading the present description.

As index information is updated, it preferably overwrites the previous version of the index information, thereby allowing the currently updated index information to be accessible at the beginning of tape in the index partition. According to the specific example illustrated in FIG. 9, a most recent version of metadata Index 3 is recorded in the index partition 902 at the beginning of the tape 906. Conversely, all three versions of metadata Index 1, Index 2, Index 3 as well as data File A, File B, File C, File D are recorded in the data partition 904 of the tape. Although Index 1 and Index 2 are old (e.g., outdated) indexes, because information is written to tape by appending it to the end of the previously written data as described above, these old indexes Index 1, Index 2 remain stored on the tape 900 in the data partition 904 without being overwritten.

The metadata may be updated in the index partition 902 and/or the data partition 904 the same or differently depending on the desired embodiment. According to some embodiments, the metadata of the index and/or data partitions 902, 904 may be updated in response to the tape being unmounted, e.g., such that the index may be read quickly from the index partition when that tape is mounted again. The metadata is preferably also written in the data partition 904 so the tape may be mounted using the metadata recorded in the data partition 904, e.g., as a backup option.

According to one example, which is in no way intended to limit the invention, LTFS LE may be used to provide the functionality of writing an index in the data partition when a user explicitly instructs the system to do so, or at a time designated by a predetermined period which may be set by the user, e.g., such that data loss in the event of sudden power stoppage can be mitigated.

A problem discovered in thin film devices including magnetic recording heads is unanticipated variation in spacing between features. Such variations may lead to various problems depending on the application.

One issue that has arisen is the necessary requirement that the spacing between the two servo elements for a given writer or reader module be precisely spaced. Due to production variations, the critical distance between servo elements can vary from one module to another. Stresses on the part during fabrication and assembly, as well as tolerances can contribute to these variations. The result of such variations can cause error in the perceived locations of where tracks are being placed. With error present, tracks are placed in an offset position from the nominal position.

Pitch variations and differences may be related to stress imbalances within the thin films themselves, perhaps arising from annealing or other processes performed on the wafers containing the head dies, and/or perhaps from mechanical and/or other post wafer processing steps used for fabricating recording head dies.

Regardless of the source of variation, there is a need for ascertaining the spans/pitches of said features. In various embodiments described herein, a span of distance between features on a sample may be determined to an accuracy of ±10 nanometers (nm) or better. Moreover, the features as described herein, enable performing precise measurement of a span between features either at the wafer level, or on finished dies, or at intermediate points of processing (wafer level, lapped surface level, finished die level, etc. Exemplary features may be portions of a thin film die (e.g. a magnetic recording die), or an entire, functional thin film device. Moreover, as described more fully below, the features may be dedicated structures of the thin film die, such that a dedicated structure is used for no other purpose.

What is needed is an efficient measurement method for characterizing individual head modules to sort those with acceptable amounts of offset between the two servo readers from those with nonacceptable amounts.

According to various embodiments described herein, precise measurements of transducer locations may be performed for one of the following outcomes, or for a combination of the following outcomes: a) diagnosing root cause of the misregistration of the transducers after processing, b) calibrating the deviation from the defined distance of the span between transducers, c) screening fabricated dies to a specification, and d) controlling the use of the head having a deviation between transducers. Precision measurements on the order of 10 nanometers or better are desired for features that may have a distance span of up to several millimeters between each feature.

Various embodiments described herein solve a problem of accurately measuring span of transducers, including the span of servo readers.

An efficient methodology is described herein that can assist in characterizing the servo to servo reader spacing for individual write and/or read head modules. This method is improved over a visual check that uses a moving stage and single video camera because, for example, various methods described herein can measure two servo readers at the same time. A camera and stage type test requires much more time, on the order of 5 minutes per module, to measure and visually confirm the servo reader positions at sub-micron positioning, and moreover, requires the stage to move the relatively long distance, e.g., approximately 3 mm between the two servo readers. For a three-module head, the time to inspect the head can be greater than 15 minutes. In contrast, an approach described herein would require a stage to move only a few microns, and be able to measure both elements the same time.

FIGS. 10 and 11 each illustrate a pair of modules. As an option, one or both modules may be implemented in conjunction with features from any other embodiment listed herein, such as those described with reference to the other FIGS. Of course, however, such modules and others presented herein may be used in various applications and/or in permutations which may or may not be specifically described in the illustrative embodiments listed herein. Further, one or both modules presented herein may be used in any desired environment.

The first module 1002 includes a pair of writer elements 1004 that are fixed at substantially the same span 1006 as a nominal servo reader spacing specified for modules that are in production for product. The writer elements 1004 may be conventional writers, or specialty writers, precisely located so that the span 1006 matches the intended “nominal” (design) servo reader spacing as closely as possible.

The writer elements 1004 may be positioned on a common substrate as shown in FIG. 10.

In another approach, the writer elements 1004 are each positioned on a portion of a unique module, the portions being adhered together, as shown in FIG. 11. For example, the writer elements 1004 may each be a part of a respective production level writer module cut and adhered to one another to provide the desired span 1006.

The span 1006 and nominal servo reader spacing may be based on center to center spacing, edge to edge spacing, or any other manner that provides an accurate measurement.

The second module 1010 may be any type of module having servo readers 1012 intended to have the nominal servo reader spacing. For example, the second module 1010 may be any module usable in a configuration shown in FIGS. 2A-7. The servo readers 1012 may be part of a transducer array 1014 that may also include reader elements, writer elements, or both. In practice, the second module 1010 may be a production level module that will eventually be used in a tape drive.

FIGS. 12A-12C graphically depict a method for measuring servo to servo reader spacing in a module, in accordance with one embodiment. As an option, the present method may be implemented to measure structures such as those shown in the other FIGS. described herein. Of course, however, this method and others presented herein may be used to provide applications which may or may not be related to the illustrative embodiments listed herein. Further, the methods presented herein may be carried out in any desired environment. Moreover, more or less operations than those shown in FIGS. 12A-12C may be included in the method, according to various embodiments. It should also be noted that any of the aforementioned features may be used in any of the embodiments described in accordance with the various methods.

Referring to FIG. 12A, the modules 1002, 1010 are positioned so that the writer elements 1004 face, and are in close proximity to, the transducer array 1014 of the second module 1010. In this example, the modules 1002, 1010 of FIG. 10 are rotated inwardly toward one another 90 degrees each.

In FIG. 12A, the writer elements 1004 are misaligned with the servo readers 1012. The two writer elements 1004 are energized with a predefined signal to cause each writer to emit measurable electro-magnetic signal. Each signal may be any type of signal, so long as it is measurable by the servo readers 1012. Exemplary signals include monotone, varying frequencies in a predefined pattern, transient frequencies, etc.

As illustrated by the transition from FIGS. 12A to 12C, the relative position between the writer elements 1004 and the transducer array 1014 is changed. The movement(s) between relative positions may be made incrementally, continuously, or a combination of both. The first module 1002 may be moved relative to the second module 1010 in the direction of the arrow shown in FIG. 12B. In another approach, the second module 1010 may be moved relative to the first module 1002. In a further approach, both modules move relative to one another. In a preferred embodiment, one of the modules is mounted to a stage of known type that provides controlled movement of the module relative to the other module.

The strength of the electro-magnetic signal detected by each servo reader 1012 at the various relative positions is measured, and optionally output for use in determining whether the servo reader span matches the nominal span, calculating the servo reader span, etc. Using the measured strengths, a determination can be made as to whether the span of the servo readers matches the nominal servo reader spacing (which equals the span 1006 of the writer elements 1004).

In one approach, the servo reader signals are characterized for signal strength at various relative locations. The relative location(s) where the maximum signal strength is observed correspond to the locations of the servo readers 1012. Accordingly, the relative position(s) where the electro-magnetic signal strength detected by each of the servo readers is at a maximum level is determined, the maximum level corresponding to a relative position of closest approach between each writer element and the respective servo reader.

For example, if both of the servo reader responses to the signals from the energized writer elements reflect detection of the maximum level electro-magnetic signal strength at the same relative position, then it can be inferred that the servo reader to servo reader span matches the nominal servo reader spacing. An indication of such inference may be output.

If one servo reader output exhibits a peak detection at one particular relative position, and the other servo reader output exhibits a peak detection at another particular relative position, then it can be inferred that the servo to servo span is not at nominal, but has an error. An indication of such inference may be output.

Moreover, by comparing the relative position(s) where the signal strength is maximally detected by each servo reader to the moving stage position, a determination can be made to approximate the actual servo reader span using known mathematical techniques. Accordingly, the higher the precision of the stage, the more accurate the calculation of the actual servo reader span. The deviation of the servo reader span from the nominal span may be programmed into a drive using that particular module to minimize and/or correct for the effects of the deviation, e.g., using known techniques.

Preferably, the processes described herein are performed in controlled environments to minimize variables such as thermal expansion, etc.

A computing circuit and logic for controlling the relative movement, energizing the writer elements, processing the outputs of the servo readers and/or performing any of the calculations is provided according to one embodiment. An electrical connection to each of the writer elements 1004 and the servo readers 1012 is also provided. Any technique known in the art for effecting the electrical connection may be used, such as using cabling. Moreover, in some approaches, the final cabling for in-drive use may be coupled to module 1010 and used in a characterization process described herein.

The present invention may be a system, a method, and/or a computer program product at any possible technical detail level of integration. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, configuration data for integrated circuitry, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++, or the like, and procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

These computer readable program instructions may be provided to a processor of a computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be accomplished as one step, executed concurrently, substantially concurrently, in a partially or wholly temporally overlapping manner, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

Moreover, a system according to various embodiments may include a processor and logic integrated with and/or executable by the processor, the logic being configured to perform one or more of the process steps recited herein. By integrated with, what is meant is that the processor has logic embedded therewith as hardware logic, such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), etc. By executable by the processor, what is meant is that the logic is hardware logic; software logic such as firmware, part of an operating system, part of an application program; etc., or some combination of hardware and software logic that is accessible by the processor and configured to cause the processor to perform some functionality upon execution by the processor. Software logic may be stored on local and/or remote memory of any memory type, as known in the art. Any processor known in the art may be used, such as a software processor module and/or a hardware processor such as an ASIC, a FPGA, a central processing unit (CPU), an integrated circuit (IC), etc.

It will be clear that the various features of the foregoing systems and/or methodologies may be combined in any way, creating a plurality of combinations from the descriptions presented above.

It will be further appreciated that embodiments of the present invention may be provided in the form of a service deployed on behalf of a customer.

The inventive concepts disclosed herein have been presented by way of example to illustrate the myriad features thereof in a plurality of illustrative scenarios, embodiments, and/or implementations. It should be appreciated that the concepts generally disclosed are to be considered as modular, and may be implemented in any combination, permutation, or synthesis thereof. In addition, any modification, alteration, or equivalent of the presently disclosed features, functions, and concepts that would be appreciated by a person having ordinary skill in the art upon reading the instant descriptions should also be considered within the scope of this disclosure.

The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 

What is claimed is:
 1. A computer-implemented method for measuring servo to servo reader spacing in a module, the method comprising: emitting a measurable electro-magnetic signal from two writer elements that are fixed at a same span as a nominal servo reader spacing; positioning the writer elements in multiple relative positions along a transducer array of a module having servo readers in the transducer array; measuring the strength of the electro-magnetic signal detected by each servo reader at the multiple relative positions; and determining, from the measured strengths, whether the servo reader span matches the nominal servo reader spacing.
 2. The method as recited in claim 1, comprising determining the relative position(s) where the electro-magnetic signal strength detected by each of the servo readers is at a maximum level, the maximum level corresponding to a relative position of closest approach between each writer element and the respective servo reader.
 3. The method as recited in claim 2, comprising determining that the servo reader span matches the nominal servo reader spacing in response to both servo readers detecting the maximum level electro-magnetic signal strength at the same relative position.
 4. The method as recited in claim 2, comprising determining that the servo reader span does not match the nominal servo reader spacing in response to the servo readers detecting the maximum level electro-magnetic signal strength at different relative positions.
 5. The method as recited in claim 4, comprising calculating the servo reader span based on the different relative positions.
 6. The method as recited in claim 1, wherein the two writer elements are positioned on a single substrate.
 7. The method as recited in claim 1, wherein the two writer elements are each positioned on a portion of a unique module, the portions being adhered together.
 8. The method as recited in claim 1, wherein the writer elements are positioned in the multiple relative positions along the transducer array of the module incrementally.
 9. The method as recited in claim 1, wherein the writer elements are positioned in the multiple relative positions along the transducer array of the module continuously.
 10. An apparatus, comprising: a processor; and logic integrated with the processor, executable by the processor, or integrated with and executable by the processor, the logic being configured to: cause two writer elements that are fixed at a same span as a nominal servo reader spacing to emit a measurable electro-magnetic signal; position the writer elements in multiple relative positions along a transducer array of a module having servo readers in the transducer array; measure the strength of the electro-magnetic signal detected by each servo reader at the multiple relative positions; and determine, from the measured strengths, whether the servo reader span matches the nominal servo reader spacing.
 11. The apparatus as recited in claim 10, comprising determining the relative position(s) where the electro-magnetic signal strength detected by each of the servo readers is at a maximum level, the maximum level corresponding to a relative position of closest approach between each writer element and the respective servo reader.
 12. The apparatus as recited in claim 11, comprising determining that the servo reader span matches the nominal servo reader spacing in response to both servo readers detecting the maximum level electro-magnetic signal strength at the same relative position.
 13. The apparatus as recited in claim 12, comprising determining that the servo reader span does not match the nominal servo reader spacing in response to the servo readers detecting the maximum level electro-magnetic signal strength at different relative positions.
 14. The apparatus as recited in claim 13, comprising calculating the servo reader span based on the different relative positions.
 15. The apparatus as recited in claim 10, wherein the two writer elements are positioned on a single substrate.
 16. The apparatus as recited in claim 10, wherein the two writer elements are each positioned on a portion of a unique module, the portions being adhered together.
 17. The apparatus as recited in claim 10, wherein the writer elements are positioned in the multiple relative positions along the transducer array of the module incrementally.
 18. The apparatus as recited in claim 10, wherein the writer elements are positioned in the multiple relative positions along the transducer array of the module continuously.
 19. A computer program product for measuring servo to servo reader spacing in a module, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a computer to cause the computer to: cause, by the computer, two writer elements that are fixed at a same span as a nominal servo reader spacing to emit a measurable electro-magnetic signal; position, by the computer, the writer elements in multiple relative positions along a transducer array of a module having servo readers in the transducer array; measure, by the computer, the strength of the electro-magnetic signal detected by each servo reader at the multiple relative positions; and determine, by the computer, from the measured strengths, whether the servo reader span matches the nominal servo reader spacing.
 20. The computer program product of claim 19, comprising program instructions for causing the computer to determine the relative position(s) where the electro-magnetic signal strength detected by each of the servo readers is at a maximum level, the maximum level corresponding to a relative position of closest approach between each writer element and the respective servo reader. 